Optical coherence tomography (abbreviated OCT) is an example of an image measuring method. OCT is a technique in which interference light between a signal light via an object and a reference light is detected and a tomographic image of the object is formed. OCT is used in the medical field etc. due to the advantage that it provides high resolution images quickly and non-invasively.
The main advancement regarding this technique includes Fourier domain OCT (abbreviated FD-OCT). In FD-OCT, by means of using a Fourier transform method, measurement speed several tens to several hundred times faster may be accomplished compared to the conventional time domain OCT (abbreviated TD-OCT).
FD-OCT includes a spectral domain OCT (abbreviated SD-OCT) that spectrally resolves and detects interference light, along with a swept source OCT (abbreviated SS-OCT) that uses a swept source to obtain interference lights of various wavelengths.
Generally, the image measuring method uses electromagnetic waves etc. to acquire analog data indicating the inner morphology of the object and processes signals obtained by digitalizing the acquired data, thereby imaging the object. The image measuring apparatus for realizing this is mounted with a digital signal processing system such as a data acquisition system (abbreviated DAS).
Analog signal sampling and A/D transformation are executed with a sampling clock signal as the trigger thereof. In the image measuring method using a Fourier transform method, the stability of the sampling clock signal greatly affects the measurement results. Hereinafter, the sampling clock signal may be simply abbreviated as the clock signal.
To illustrate an example, a case of FD-OCT is explained. In FD-OCT, the detected spectrum, that is, the spectral interferogram (interference spectrum) is expressed by the following formula.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                      I          ⁡                      (            k            )                          =                              s            ⁡                          (              k              )                                ·                      (                                          I                R                            +                              I                S                            +                                                ∫                                      -                    ∞                                                        +                    ∞                                                  ⁢                                  2                  ⁢                                                                                    I                        R                                            ⁢                                              I                        S                                                                              ⁢                                      cos                    ⁡                                          (                                                                        k                          ⁢                                                                                                          ⁢                          z                                                +                                                                              ϕ                            0                                                    ⁡                                                      (                            z                            )                                                                                              )                                                        ⁢                                      ⅆ                    z                                                                        )                                              (        1        )            
Where, k indicates the wave number, s(k) indicates the light source spectrum, z indicates the difference in optical-path-length between a sample arm and a reference arm, IR indicates the back reflection intensity of a reference light by a reference mirror, IS indicates the signal light intensity having passed the object, while φ0 (z) indicates the initial spectral phase term. Generally, IR and IS are low frequency signals or a background component (DC component), which may be easily removed. Thereby, the Formula (1) is simplified as follows.
                    [                  Formula          ⁢                                          ⁢          2                ]                                                                      I          ⁡                      (            k            )                          =                              ∫                          -              ∞                                      +              ∞                                ⁢                                    A              ⁡                              (                z                )                                      ⁢                          cos              ⁡                              (                                                      k                    ⁢                                                                                  ⁢                    z                                    +                                                            ϕ                      0                                        ⁡                                          (                      z                      )                                                                      )                                      ⁢                          ⅆ              z                                                          (        2        )            
Where, A(z)=s(k)·2√(IRIS) indicates the amplitude of the interference signal proportionate to the back scattering intensity of the object at a depth z. By means of performing Fourier transform of I (k) based on Formula (2), the backscattering profile of the object at depth z, that is, an A-line profile may be reconstructed.
Regarding SS-OCT, the ideal morphology of a spectral interferogram waveform corresponding to depths z=d1, d2, and d3 as well as the ideal morphology of the signals obtained from performing Fourier transform on these are illustrated in FIG. 1. Meanwhile, an example of actually acquired signals is illustrated in FIG. 2. A signal 1000 includes a signal component 1001 corresponding to actual signals and an artifact component (side peaks) 1002. The artifact component 1002 restricts the dynamic range, causing deterioration of image quality.
As illustrated in FIG. 3, a clock signal 2000 includes a timing component 2001 indicating the timing of the trigger in addition to a noise component (clock noises) 2002. The artifact component 1002, as illustrated in FIG. 2, is generated by such clock noise 2002.
The method for generating clock signals is explained. For example, in SS-OCT, dedicated clock signals are used in order to linearly sample the spectral interferogram in a k-space. A configuration for generating these clock signals is illustrated in FIG. 4.
Light output from the swept source 3010 is input into an interferometer 3030 via an optical fiber 3020. The interferometer 3030 has a specific difference in optical-path-length. That is, an optical delaying part 3033 is, for example, configured with a fiber stretcher, providing a difference in optical-path-length between a path via the optical fibers 3032 and 3034 and a path via an optical fiber 3035.
Light input into the interferometer 3030 is divided into two by a fiber coupler 3031. One of the lights is lead to the optical delaying part 3033 via the optical fiber 3032 to receive a specified amount of phase delay, and then lead to the fiber coupler 3036 via the optical fiber 3034. The other light is lead to the fiber coupler 3036 via the optical fiber 3035. The fiber coupler 3036 causes the two lights to interfere. The generated interference light comprises a frequency corresponding to the amount of the phase delay. This interference light is detected by the photodetector 3050 via the optical fiber 3040.
Signals output from the photodetector 3050 are input to an amplifier 3070 via the signal line 3060. The amplifier 3070 amplifies the output signals of the photodetector 3050. The signals output from the amplifier 3070 are the clock signals 4000 illustrated in FIG. 5. The clock signals 4000 are used as the trigger signals for linear sampling of the spectral interferogram in the k-space.
As mentioned above, the interferometer 3030 comprises a constant difference Δz in optical-path-length. The difference Δz in optical-path-length is set in accordance with a request from the sampling resolution. The sampling resolution Δk is defined as the inverse number of the difference Δz in optical-path-length: Δk=1/Δz.
Moreover, the clock signal Sn corresponding to a sampling point is expressed as follows:Sn=A·cos(2π·Δz·kn+φ)=A·cos(2π·n+φ)=A·cos(φ).
As can be seen from this formula, the sampling of the spectral interferogram is preferably performed at the point where the instantaneous phase of the clock signal Sn becomes constant. Particularly, taking into consideration the fact that there are no affects due to signal amplitude and light intensity, it is believed that a zero-cross point of the clock signal is most preferable. Symbols 4010j (j=1, 2, 3, . . . ) in FIG. 6 illustrate the zero-cross points of the clock signal 4000. Such zero-cross points 4010j are zero-cross points at the rising edge of the clock signal 4000. In such case, the value of φ in the clock signal Sn becomes φ≡3π/2.